Field of the Invention
The present invention relates to an open-type X-ray tube comprising a field emission type electron gun and an X-ray inspection apparatus using the same, and in particular to a high-resolution type X-ray tube and an X-ray inspection apparatus using the same, whose performances are stable by improving a workability of the baking.
Description of Related Art
With regard to uses of study, development and analytical investigation and so on of biological samples, solid-state materials, industrial electronic parts or the like, inspection apparatuses (hereinafter, merely called “X-ray inspection apparatus(es)”), such as a projection type X-ray inspection apparatus, an X-ray CT apparatus and so on, are known as the apparatuses for observing internal condition and structure of samples at non-destructive and high-resolution. In this high-resolution type (it is operable for the performance to discriminate the sample at a size from few micrometers to a few dozen nanometers.) X-ray inspection apparatus, an X-ray tube having a micro-focus diameter and high-intensity X-ray source is essentially necessary.
The X-ray tube used in such use has an open-type tube constitution. That is, the electron beam emitted from a thermal electron gun (using LaB6 (Lanthanum hexaboride) as the electron source), a field emission type electron gun or a thermal field emission type electron gun (generally called “Schottky-type electron gun(s)”) is accelerated with an electrical field, then the electron beam is focused to a beam of extremely micro-focus diameter by using an electron lens, and then the focused beam is irradiated on the X-ray target, thereby to generate the micro-focus diameter and high-intensity X-ray source at the X-ray target. Then, although a sealed type X-ray tube using materials, such as glass and ceramics and so on, for vacuum sealing is generally used, the high-resolution type tube comprising the field emission type electron gun is not put to practical use in the sealed type X-ray tube. That's why it is difficult to maintain the inner of the X-ray tube in ultra-high vacuum condition.
In the projection type X-ray inspection apparatus, a transmission X-ray image is obtained by detecting with an X-ray detector while geometrically enlarging the X-ray image transmitted from the sample by irradiating an X-ray beam generated from the X-ray source to the sample (e.g. refer to Non-Patent Document 1).
In the X-ray CT apparatus, in addition to the above-mentioned constitution, the transmission X-ray images from plural different directions relative to the sample are obtained by rotating the sample relative to the X-ray beam, and then a three-dimensional structural information of the inner of the sample are obtained by generating a set of computed tomographic images due to an image-reconstruction processing of the plural transmission X-ray images. In case of using the X-ray tube that has the X-ray source that a focus diameter is equal to or less than 400 nm, the X-ray CT apparatus having 150-200 nm of the resolution is known (e.g. refer to Non-Patent Document 2).
In order to obtain good X-ray images in the X-ray inspection apparatus, it is necessary to generate a stable X-ray beam by emitting a stable electron beam from the electron source of the X-ray tube. To that end, it is important to keep an ultra-high vacuum condition for a long term by well ultra-highly evacuating (107˜10−8 Pa) a surrounding area of the electron source of the electron gun. That's why that an emission of electron is unstable, the electron emission characteristic turns down, and the lifetime of the electron source decreases, so gas molecules adsorb on a pointed end portion of the electron source of the field emission type electron gun or the pointed end portion receives an ion bombardment by the gas molecules unless the ultra-high vacuum condition is stable at the surrounding area of the electron source.
In the general art, as shown in FIG. 1, by vacuum-separating a traveling space of an electron beam 103, from an electron gun 100 comprising a Schottky-module 100a, an electron source 100b, an anode 100c, a magnetic-field superposed lens 100d and an electron beam axis alignment coil 100e etc. to an X-ray target 101, from magnetic circuit etc. of an electro magnetic lens (objective lens) 104 prone to gas, it is known to form the surrounding area of the electron source 100b of the electron gun 100 to an ultra-highly vacuum (e.g. refer to Japanese Unexamined Patent Publication No. 2004-138460 A (Patent Document 1)). As well, in FIG. 1, “112” shows an X-ray, the X-ray 112 transmits a sample (object to be inspected), and the transmitted image(s) is/are detected by an X-ray detector 111.
In the open-type X-ray tube, it is well known to use a liner tube (pipe) for the electron traveling path of the electron beam. The liner tube is held by using an elastomer O-ring, which is an airtight sealing member, between an electron gun chamber and a pointed end member including the X-ray target. The airtightness ensures with the above constitution, and the vacuum of the space among the electron gun˜the liner tube˜the X-ray target is held (e.g. refer to Japanese Unexamined Patent Publication No. 2009-301908 A (Patent Document 2)). Although general electron microscopes also have similar constitutions, the liner tube of the open-type electron tube has an easily detachable constitution in a single body from the main body of the apparatus and is able to carry out the periodical cleaning of the inner wall or the like of the liner tube.
In the X-ray inspection apparatus having the open-type X-ray tube, it is necessary to put the inner of the electron gun chamber in air in the works, such as manufacturing and assembling of the apparatus, change of the electron source nearly and routinely implemented, maintenance of the inner of the electron gun chamber, and so on. Once, to return the electron gun chamber put in air to the ultra-high vacuum condition, the work to make the X-ray tube baking again is necessary. The baking means the work to release the various gas molecules, which are adsorbed or absorbed in a metal surface or inner of a member facing the vacuum of the electron tube apparatus, from the metal surface. By sufficiently baking out before using the apparatus, it is easily operable to evacuate to the ultra-high vacuum condition in a use time. Because it uselessly takes a long time to evacuate unless the baking is sufficient, the required ultra-high vacuum condition is not quite obtained.
It is an ideal that the work of baking is usually performed by heating the member facing the vacuum with a high temperature (200˜450° C.) and continuously evacuating for long times (about 24˜100 hours) by the vacuum pump (an ion pump, a turbo-molecular pump and so on) for the ultra-high vacuum. As the heating method, for example, it is to continuously evacuate for long times (about 24˜100 hours) by the vacuum pump for the ultra-high vacuum, while winding a sheath heater across the apparatus part around the X-ray tube and heating the X-ray tube with a high temperature (200˜450° C.) by turning on electricity to the sheath heater. By this baking, it is possible to obtain a stable and ultra-high vacuum tube section including the electron gun chamber.
The electron microscope and so on have constitutions to be able to easily vacuum-separate the electron gun chamber from an electron lens system, the sample chamber and so on. That is, it is operable to bake the only part of the electron gun chamber by using a constitution that an intermediate chamber is provided between the electron gun chamber and the electron lens system and each part is separately evacuated with a differential pumping. In this connection, the method to use the lens system thereafter in usual vacuum (10−3˜10−4 Pa), while only keeping the part of the electron gun chamber in the ultra-high vacuum (10−7˜10−8 Pa), is also much used.
On the contrary, since the X-ray inspection apparatus has the constitution which is difficult to perform the vacuum-separation by such the differential pumping, the method to entirely bake with the inclusion of the electron lens system is adopted.
As the constitution for easily performing the baking of the electron microscope, it is proposed to vacuously separate the electron gun chamber from the liner tube and so on by providing a gun valve on an exit of a field emission type electron gun chamber and closing the gun valve. It is operable to individually bake the only surrounding portion of the electron gun chamber by detaching the electron gun chamber from the main body of the electron microscope apparatus while keeping the inner of the electron gun chamber in vacuum (e.g. refer to Japanese Unexamined Patent Publication No. 2006-294481 A (Patent Document 3)).